A transmitter 100 generates radio signals required for communications. It consists of a data source, modulator 110, power amplifier (PA) 120 and antenna 130, as shown in FIG. 1. The modulator 110 varies the amplitude, phase, and/or frequency of a carrier signal depending on the communication method selected.
FIG. 2 shows a detailed diagram of a standard radio transmitter architecture 200. The transmitter comprises a digital modem 210, a pair of digital-to-analog (D/A) converters 220, a phase-locked loop circuit 240, an oscillator 250, a frequency divider 260, a pair of mixer 230, a driver 270, a SAW filter 280 and an amplifier 120. It uses two quadrature mixers 230—known as an I/Q modulator—to generate the modulated signal at the radio frequency. A SAW filter 280 usually follows the I/Q modulator to limit broadband noise and spurs. This architecture operates with reasonable performance. It also allows for reduced current consumption at low to moderate output power levels where requirements are less stringent. These relaxed requirements are illustrated for a WCDMA modulator in FIG. 3.
A polar transmitter 400 uses the approach shown in FIG. 4 to form the modulated signal, where the system comprises a digital modem 410, a variable gain amplifier (VGA) 420 and PA 120. This modulator better suppresses wideband noise—making it possible to eliminate the SAW filter. This is a key benefit. Another benefit is its lower current consumption at high output power levels. Unfortunately, its current consumption cannot be easily reduced to take advantage of relaxed requirements at low to moderate output power levels. It would therefore be advantageous to provide a radio architecture that can operate more efficiently at low to moderate output power levels.